FIG. 9 illustrates a constant on-time DC/DC converter. A direct-current input voltage Vin, which is supplied from a battery or similar source, is supplied to one end of a coil 2 via a switch circuit 1. One end of the coil 2 is connected to the cathode of a diode 3, while the anode of the diode 3 is connected to ground GND.
The other end of the coil 2 is connected to ground GND via a capacitor 4. An output voltage Vout is then output on the basis of the switching control of the switch circuit 1.
FIG. 10 is a timing waveform diagram illustrating the operation of a DC/DC converter. The output voltage Vout is input into the minus-side input terminal of a comparator 5, while a reference voltage Vref is input into the plus-side input terminal of the comparator 5. The output signal from the comparator 5 is output to a mono multivibrator circuit (MM) 6, and the switching of the switch circuit 1 is controlled by the output signal Vmm from the mono multivibrator circuit 6.
When the output voltage Vout goes lower than the reference voltage Vref, the comparator 5 outputs an H-level signal. When the output voltage Vout goes higher than the reference voltage Vref, the comparator 5 outputs an L-level signal. The mono multivibrator circuit 6 outputs an output signal that is H-level for a fixed amount of time when triggered by the rising of the output signal from the comparator 5.
When the output voltage Vout falls to or below the level of the reference voltage Vref, the output signal Vmm from the mono multivibrator circuit 6 rises to H-level, the switch circuit 1 enters a conducting state, and the output voltage Vout rises.
Once a fixed amount of time has elapsed since the rising of the output signal Vmm from the mono multivibrator circuit 6, Vmm goes to L-level. At this point, the switch circuit 1 becomes non-conducting, and supply of the input voltage Vin is cut off, which causes the output voltage Vout to fall. Once the output voltage Vout has fallen to the reference voltage Vref, the above operation is repeated. In this way, the generated output voltage Vout is smoothed by an output smoothing capacitor, and an average value Vave is supplied to an external device as power.
However, in a DC/DC converter like the above, fluctuations in the input voltage Vin fluctuates will cause the ripple (i.e., the slope of the rise and the wave height value) to change in the output voltage Vout, which causes fluctuations in the average value Vave supplied to the external device.
FIG. 11 illustrates another constant on-time DC/DC converter (U.S. Pat. No. 5,770,940). The output voltage Ve from an error amplifier 7 is input into the plus-side input terminal of a comparator 5. The output voltage Vout is input into the minus-side input terminal of the error amplifier 7 via a resistor, while a reference voltage Vref is input into the plus-side input terminal.
The error amplifier 7 operates so as to lower the output voltage Ve when the output voltage Vout goes higher than the reference voltage Vref, and to raise the output voltage Ve when the output voltage Vout goes lower than the reference voltage Vref. If the output voltage Vout rises, the rise timing of the output signal from the comparator 5 occurs later. If the output voltage Vout lowers, the rise timing of the output signal from the comparator 5 occurs earlier.
As a result, the above DC/DC converter operates such that the average value Vave of the output voltage Vout converges on the reference voltage Vref, and the fluctuations in the average value Vave due to fluctuations in the input voltage Vin are suppressed.